1. Field of the Invention
The invention relates to a reproducible standard for calibrating and checking the bright field channel of a surface inspection device which is used for examinations of a smooth surface of a specimen. The invention also relates to a method for making the standard
2. The Prior Art
Smooth surfaces of a specimen, for example the sides of semiconductor wafers, are customarily examined using surface inspection devices, the surface to be examined being automatically scanned using a laser beam. Defects on the surface of the specimen, in particular raised and lowered portions of materials, and also foreign particles, can be identified and quantified by detecting and evaluating the light scattered by the surface of the specimen. For measurements in the dark field channel of the surface inspection device, that is to say for measurements of the scattered light produced non-specularly when the laser beam strikes, internationally recognized standards are available, using which the dark field channel of the surface inspection device can be calibrated and checked. Such a standard consists, for example, of a polished semiconductor wafer, on one of whose sides a particular number of microspheres with particular average size and size distribution are deposited.
The microspheres represent only a rough approximation of the defects which are actually to be found on smooth surfaces. Furthermore, especially defects which, over a comparatively large lateral extent, span only small height differences, are not detected in the dark field channel. They can, however, be picked up in the bright field channel of the surface inspection device, that is to say by measuring the laser light reflected specularly from the surface of the specimen. They are picked up customarily by measuring intensity losses of the laser beam reflected in the specular direction or by measuring the differential interference contrast ("Normaski contrast"). However, there is currently no reliable standard with which the bright field channel of the surface inspection device can be checked and calibrated. Standards which are based on substrates that have microspheres applied to them are found to be unsatisfactory for examinations in the bright field channel. A bright field standard of this kind must have comparatively large microspheres applied to it. Since the adhesion of microspheres to a substrate decreases as the sphere diameter increases, it is difficult to work with the standard. Furthermore, such a standard is not cleanable. It has also been found inappropriate to use known step-height standards as standards for measurements in the bright field channel. This is because, when such standards are used, no evidence of edge gradients of defects is encountered, so that realistic defect characterization is not possible.